1. Technical Field
The present invention relates to a method of producing magnesium-based hydrides that generate hydrogen when reacted with water, more particularly, to a method of producing magnesium-based hydrides and an apparatus for producing magnesium-based hydrides, capable of producing magnesium-based hydrides from materials at high yield.
2. Description of the Related Art
Like an alkali metal hydride, magnesium hydride MgH2 is an ionic bond hydride formed of the bonding of Mg2+ and H− and is formed by reacting magnesium Mg with high pressure hydrogen under heating. The reaction for forming MgH2 is represented by the following formula (1).Mg+H2→MgH2  (1)
The powder of MgH2 is charcoal gray in color and stable in air. However, when it is in water, it reacts with water and decomposes while releasing hydrogen. Hence, it is expected that MgH2 can be used as a hydrogen storage material that releases hydrogen by causing MgH2 to store hydrogen in advance and by reacting MgH2 with water as necessary. The reaction of MgH2 with water is represented by the following formula (2).MgH2+2H2O→Mg(OH)2+2H2  (2)
The powder of Mg reacts with hydrogen in the temperature range of 250 to 400° C. and is hydrogenated gradually. It is thus difficult to completely hydrogenate Mg by carrying out heating and high-pressure treatment only once. For this reason, in order that the hydrogenation of Mg is accelerated, Mg is hydrogenated to some extent to cause Mg to absorb hydrogen. Then, Mg is heated once under reduced pressure to release hydrogen. This hydrogen absorption/release operation is repeated ten or more times. The “activation treatment” described above is usually necessary for the hydrogenation.
Furthermore, usually, the surface of Mg is oxidized, and an oxide coating of MgO or Mg(OH)2 or a nitride coating is formed thereon, whereby the initial hydrogen-absorption rate of Mg is low. Hence, a method of improving the efficiency for Mg to absorb hydrogen by adding a substance having an effective catalytic action for initial activation and hydrogen absorption to Mg has been proposed. As substances having the catalytic action, I2, Ni, Cu, etc. are known. In addition, Patent document 1 has disclosed an art of improving the efficiency of absorbing hydrogen by causing the surface and inside of granular Mg to contain metal particles having the catalytic action, such as Ni particles.
[Patent document 1] Japanese Patent Application Laid-Open Publication No. 2003-212501
However, even when MgH2 is produced from Mg while the activation treatment is carried out, since the diffusion rate of hydrogen in Mg is low, it is difficult to produce high purity MgH2. A small amount of MgH2 having a purity of approximately 95% is produced at present. The longer the activation treatment is continued, the more the yield of MgH2 is improved. However, the energy supplied to produce MgH2 increases, and there is a problem that the energy cost of MgH2 is high. Furthermore, although the yield of magnesium-based hydrides containing MgH2 is improved by adding the substance having the catalytic action to Mg, the recycle treatment of waste after hydrogen is generated from the magnesium-based hydrides becomes difficult. Hence, it is desirable that the amount of the additive should be small.